The present invention relates to a reproduction system having a color image processing apparatus for calculating tristimulus values under observation illumination light corresponding to the spectral reflectance of an object.
In recent years, color management systems (CMS) for matching the colors of input/output images between a plurality of color image devices such as a color CRT monitor and color printer are becoming popular in various fields handling color images.
In a CMS, a problem is arisen in accurately reproducing colors to be perceived when sensing illumination light in sensing an object is different from observation illumination light in observing an image obtained by displaying image data obtained by sensing on a CRT monitor or outputting it by a printer because the colors of the observed image are different between these environments due to the change in human visual characteristics.
As a solution to this problem, a color reproduction method is employed in which the spectral reflectance f(λ) of the object is obtained, and CIE XYZ values of the reflectance spectrum are calculated by multiplying the spectral reflectance by the spectrum Eo(λ) of observation illumination light for display.
The spectral reflectance f(λ) is obtained in correspondence with each pixel of the object image obtained by sensing the object. The CIE XYZ values are calculated by equations (1) using XYZ color matching functions x(λ), y(λ), and z(λ) defined by the International Commission on Illumination (CIE).       X    =                  ∫                  λ          =          380                780            ⁢                        x          ⁡                      (            λ            )                          ⁢                              E            o                    ⁡                      (            λ            )                          ⁢                  f          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                      ⁢              Y    =                  ∫                  λ          =          380                780            ⁢                        y          ⁡                      (            λ            )                          ⁢                              E            o                    ⁡                      (            λ            )                          ⁢                  f          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                      ⁢              Z    =                  ∫                  λ          =          380                780            ⁢                        z          ⁡                      (            λ            )                          ⁢                              E            o                    ⁡                      (            λ            )                          ⁢                  f          ⁡                      (            λ            )                          ⁢                  ⅆ          λ                      ⁢        where λ is the wavelength [nm]. In this method of obtaining the spectral reflectance f(λ) corresponding to each pixel of the object image and calculating the XYZ values under the observation illumination light on the basis of the spectral reflectance for display, the XYZ values when the object is present under the observation illumination light are displayed. For this reason, accurate colors to be perceived can be reproduced without using a visual model for chromatic adaptation.
In the above-described conventional color reproduction system, to calculate the XYZ values under the observation illumination light from the spectral reflectance f(λ) of the object, the spectrum Eo(λ) of the observation illumination light must be obtained.
However, to accurately measure the spectrum, a measurement instrument such as a spectrophotometer is necessary. Since spectrophotometers must be prepared at various observation positions to measure the spectrum of the observation illumination light Eo(λ), resulting in cumbersomeness.